1. Field of the Invention
This invention relates to a filtering method of flue gas emitted from a boiler such as a pressurized fluidized bed combustion boiler or a coal direct firing boiler, employing a filter apparatus integrated with plate-like filters or filter tubes made mainly of ceramics such as of mullite species, cordierite species, silicon carbide species or the like, and a filter apparatus for hot gas preferably employed in filtering flue gas of a boiler.
2. Discussion of the Related Art
A filter apparatus integrated with ceramic filters has already been reduced to practice for filtering flue gas at an elevated temperature of a low-level radioactive waste incinerator. However, the dust concentration of the flue gas is small and the variation of a combustion load thereof is extremely small. In other words, currently, it has been reduced to practice for an extremely limited usage.
On the other hand, the filter apparatus integrated with ceramic filters, is considered to be a component technology or a key technology for reducing to practice a coal gasifier power generating plant or a power generation plant employing a pressurized fluidized bed combustion boiler, which is a clean (wherein the emitted flue gas is composed of low NO.sub.x and SO.sub.x) energy technology of the next generation employing coal as fuel. Currently, intensive effort has been paid for the practical achievement of such a generation plant in various countries in the world.
In such a kind of power generation plant, power is generated by a combined cycle in two stages wherein power is generated by steam provided by a boiler and by driving a gas turbine by cleaned flue gas, which is obtained by removing dust from the flue gas after combustion under a pressurized state. Therefore, a power generating efficiency thereof is high. Since lime or dolomite is charged into the fluidized bed along with coal, almost no sulfur oxide (SO.sub.x) is generated. Since a combustion temperature thereof is restrained to be equal to or below approximately 900.degree. C., the generation of nitrogen oxide (NO.sub.x) is in a small quantity.
In employing the filter apparatus integrated with ceramic filters in filtering hot flue gas of a power generation plant or the like, it is necessary that the filter apparatus correspond to various operating conditions of the whole plant. Especially, in filtering flue gas emitted from a pressurized fluidized bed combustion boiler employing coal as fuel, when it is operated to increase the boiler load, combustion oxygen in the boiler becomes deficient and a large amount of unburnt component including soot is emitted.
In case of a bubbling bed type pressurized fluidized bed combustion boiler, when a load thereof varies under a low-load condition wherein a bed height of the fluidized bed is low, even if the oxygen concentration is high, a large amount of unburnt component is emitted in the flue gas, and carbon monoxide having a concentration exceeding 1000 ppm is often contained in the flue gas. Under such a situation, the concentration of dust in the flue gas increases 5 to 10 times as much as that in a steady state, by flying-over of the unburnt component, and a ratio of the unburnt component such as soot contained in the dust may be not smaller than 30%.
In the filtering of flue gas in the pressurized fluidized bed combustion boiler, it is the general tendency to adopt a system which prevents erosion of a gas turbine by dust, by cleaning flue gas which has been pre-cleaned by a cyclone at a prestage by a filter apparatus integrated with ceramic filters. However, in case of the cyclone, in a transient state wherein the dust concentration of the flue gas rapidly increases, the dust often overflow and an effective filtering function cannot be performed.
Moreover, the dust containing a large amount of unburnt component which pass through the cyclone, reaches ceramic filters of the filter apparatus. At this occasion, the dust concentration in the flue gas become 5 to 10 times as much as that in a steady state, and an increasing rate of a filtering pressure difference becomes 5 to 10 times as much as that in a steady state. In such a case, the operating conditions of the filter apparatus become unstable and finally the filtering function of the filter apparatus is lost.
Furthermore, there is a case wherein fuel is purged when the operation of the boiler is stopped, thereby blowing fuel which remains in a fuel charge manifold into the boiler. When this operation is performed just after a low-load operation, a large amount of coal powders are blown instantaneously through the pressurized fluidized bed combustion boiler, through the cyclone and reach the ceramic filters of the filter apparatus and the dust concentration in the flue gas reaches a value 100 to 300 times as much as that in the steady state.
In many tests in the filter apparatus wherein the filtering of flue gas in the pressurized fluidized bed combustion boiler is tested, the evaluation has been carried out under the condition wherein there is almost no variation of the boiler load. The reason is because the structure of the ceramic filters and the filter apparatus cannot stand the increase of filtering pressure difference by the dust laden gas having a high dust concentration containing unburnt component, generated by the variation of the boiler load, or a temperature variation caused by combustion of the unburnt component which takes place occasionally.
For instance, in a filter apparatus wherein ceramic filter tubes, each of which one end is closed, are arranged in a candle (candle stand) -like form (hereinafter candle type filter apparatus), disclosed in U.S. Pat. No. 4,904,287 and U.S. Pat. No. 5,059,227, since there is a problem in a supporting structure of a tube sheet made of a refractory alloy, when strong reverse cleaning is performed, the tube sheet and the filter tubes separately vibrate, the filter tubes or sealing parts thereof are damaged and the dusts leaks to the downstream side. Therefore, the reverse cleaning conditions are not determined by the necessary intensity of the reverse cleaning, but by the structural restrictions. Accordingly, when the filtering pressure difference becomes large, the reverse cleaning function becomes deficient, and as a result, the filtering becomes impossible due to the increase of the filtering pressure difference.
As another reason wherein a strong reverse cleaning of the filter cannot be adopted in the candle type filter apparatus, it is pointed out that the tube sheet is not cooled. Normally, the temperature of the flue gas emitted from the pressurized fluidized bed combustion boiler is in a range of 800.degree. to 950.degree. C. Therefore, creep of the tube sheet takes place when the filtering pressure difference becomes large even if the tube sheets made of a refractory alloy are employed.
Furthermore, since it is difficult to regulate flow in the space of the dust laden gas side, or the outside of the filter tubes into a smooth down-flow, there is a tendency wherein the dust adheres to and accumulates partially on the surface of the filter tubes in a large amount. When the adhered and accumulated unburnt component is ignited by any chance, it is combusted and serious damage is caused on the filter tubes.
Furthermore, in a filter apparatus provided with cross-flow filters having a structure of piled-up ceramic filter plates (hereinafter cross-flow type filter apparatus) described in Japanese Unexamined Patent Publication No. 198606/1990, when flue gas having a high dust concentration is to be filtered, gas passages are liable to be blocked by the dust. When strong reverse cleaning is necessary, the strength of the cross-flow filters is structurally insufficient. Accordingly, this device does not achieve a technical level wherein it can be operated under a variety of a boiler load.
In a filter apparatus having passages of dust laden gas penetrating inside of ceramic filter tubes (hereinafter tube type filter apparatus) disclosed in U.S. Pat. Nos. 4,584,003, 4,753,457, 4,867,769, 5,073,178 and the like, it is structurally possible to perform strong reverse cleaning. The filter apparatus is possible to be employed in a wide variety of operating conditions including those in a pressurized fluidized bed combustion boiler, by utilizing silicon carbide species or cordierite species ceramics for filter tubes.
In the tube type filter apparatus, since the inside of the filter tubes are passages for the dust laden gas, and the pressure difference of reverse cleaning gas is applied from outside of the filter tubes, mainly compressive stress is applied on the ceramics, and the pressure difference of the reverse cleaning does not help promote the damage of the filter tubes. Furthermore, the tube type filter apparatus is provided with the construction capable of achieving a large capacity by employing the filter tubes connected in the vertical direction and by piling up cleaned gas chambers. Since the dimensions of the tube sheets are comparatively small, the tube sheets can be cooled, if it is required. When the cooling is effective, the tube sheets do not creep even when strong reverse cleaning is performed. Therefore, it is possible to perform the strong reverse cleaning sufficiently. The apparatus can be employed under wide ranges of conditions of temperature, pressure and dust concentration.
That is to say, in the tube type filter apparatus, even when the dust concentration increases to some degree, it is possible to correspond thereto by previously designing the apparatus such that the strong reverse cleaning can be performed. Furthermore, in this filter apparatus, the flow of the dust laden gas is completely a down-flow and the flow area of the dust laden gas in the filter chamber is small, the dust is hard to be accumulated thickly on the filter, so long as an absolute quantity of the entrained unburnt component does not vary considerably. For instance, there causes no problem of thermal damage of the filter tubes or the like, so far as the unburnt component of approximately under 10% is always contained in the dust and the unburnt component combusts little by little.
However, even in the tube type filter apparatus, when a large amount of unburnt component flows into the apparatus in a short while, the dusts thickly adheres to and is accumulated on inner faces of lower end portions of the filter tubes wherein the down-flow velocities are almost null, the accumulated dust is ignited and combust in a short time, and temperatures at the portions of the filter tubes are elevated rapidly by the combustion heat. At this occasion, the temperature difference between the inside and the outside of the filter tubes exceeds an allowable temperature difference, and the filter tubes are destroyed by the thermal stress.
Furthermore, since the viscosity of gas is large at an elevated temperature, the dust in the hot gas floats against the gravity on the flow of the hot gas. Even when the main flow of the dust laden gas in a filter chamber (a space wherein solid-gas separation is performed in the filter apparatus) is constructed to be approximately a down-flow, a portion of fine dust floats against the gravity and is hard to drop down to the bottom of a hopper.
In an actual plant, flow of dust laden gas in the filter chamber constantly fluctuates by disorder of the dust laden gas flow caused by the operating conditions of the filter apparatus per se, or by disturbances at an upstream system or a downstream system of the plant, the turbulent flow of the dust laden gas accompanied by various secondary flows is constantly present in the filter chamber.
As a result, various phenomena causing the trouble mentioned below are generated in the filter chamber. One of the phenomena is bridging of dust, which is often observed in the candle type filter apparatus or the cross-flow type filter apparatus. By the presence of a transverse secondary flow or an upward secondary flow, even if the reverse cleaning of the ceramic filter is performed, the dust does not drop to the bottom of the hopper, and adhere again to the nearby ceramic filters. Finally, a portion of the flow passages on the side of the dust laden gas of the ceramic filters, is blocked by the accumulated dust, or a portion of the surface of the ceramic filters is covered with a thick accumulated layer of dust.
In case of the tube type filter apparatus, the gas flows at the lower end portions of the filter tubes fluctuate among the filter tubes, since the gas flow is influenced by a pressure and a velocity distribution of the dust laden gas at a gas inlet chamber of the vessel. It is clarified that there is even a filter tube wherein the dust laden gas flows reversely from the hopper.
When such phenomena take place and the dust is accumulated partially on the portions of the filter tubes, an effective filtration area of the ceramic filter decreases thereby decreasing the filtering capacity of the filter apparatus. In case of filtering the flue gas of the pressurized fluidized bed combustion boiler containing the unburnt component such as soot, the unburnt component contained in the accumulated dust is often ignited and combusted, and the ceramic filters receive a serious damage by the thermal stress caused by the combustion heat.
In filtering a synthetic gas in a coal gasifier plant, when the inner atmosphere is switched from a state, wherein the inside gas is filled with nonoxidizing gas, to air instantly after the stoppage of the operation, or oxidizing gas containing oxygen is introduced in the filter apparatus when the operation is restarted, the unburnt component in the accumulated dust is ignited and combusted whereby the ceramic filter receives a serious thermal stress damage.
A tube type filter apparatus is proposed in Japanese Examined Patent Publication No. 24251/1991, wherein the dust laden gas is extracted from the hopper (called blowing down) and is recirculated to the upstream of the filter apparatus. According to this filter apparatus, the dust laden gas at an elevated temperature having a high energy level is recirculated by the blowing down without depositing the dust laden gas to the outside, a downward flow is provided in the vicinity at the lower ends of the filter tubes, and the concentrated accumulation of dust at the lower end of the filter can be avoided. Therefore, the pressure difference of filtration can be decreased as a whole and the frequency of the reverse cleaning can be decreased.
However, a proper power is necessary to recirculate the dust laden gas. It is preferable to employ an ejector in use of steam or compressed air as a driving power, as a means of transporting the dust laden gas. However, the efficiency thereof is as small as approximately 4% at the most. This method can be employed in a case wherein a small quantity of the dust laden gas is recirculated. However, in recirculating a large amount of the dust laden gas, the energy loss is considerable.
Furthermore, recirculating the dust laden gas at an elevated temperature under pressure by a blower, is not a preferable method, since the installation of the blower is not easy and the blades of the blower are eroded by the dust. The recirculation of the dust laden gas is a potential method of solving the problem even in the case of the candle type filter apparatus. However, in this type of filter apparatus, since the flow area of the dust laden gas in the filter chamber is large, it is necessary to recirculate a large amount of the dust laden gas and the reduction of the method to practice is further difficult.
The blowing down operation wherein the dust laden gas is extracted from the hopper and is deposited, is experimentally performed with the effect being the same than with that in the blowing down operation wherein the recirculation is performed. However, the influence thereof on the efficiency of the plant is considerable and the extracted amount in a power generating plant is restricted to 1 to 2% of the total at the most. Therefore, a sufficient blowing down effect cannot be provided.